In designing a diffraction grating employed in a spectrometer or a monochromator, the profile of the grooves should be designed so as to reveal the highest diffraction efficiency as much as possible. In a blazed type diffraction grating having a saw-like profile, for example, the blaze angle should be optimized, and in the laminar type diffraction grating having grooves of a rectangular profile, the depth of the grooves and the duty ratio should be optimized, to obtain the highest diffraction efficiency.
When such a diffraction grating is employed in a constant deviation angle monochromator in which wavelength scanning is done by rotating a diffraction grating about an axis parallel to the grooves of the diffraction grating, however, the diffraction efficiency is maximized only at a certain wavelength but is not maximized at the other wavelengths. Two methods have been proposed addressing the problem. In one of the methods, auxiliary mirrors are used to change the deviation angle according to the wavelength (M. Koike, “High resolution EUV monochromator/spectrometer,” U.S. Pat. No. 5,528,364). In the other method, the depth of the grooves of a laminar type diffraction grating is varied along the length of the groove, as shown in FIG. 5. When the diffraction grating is rotated about the rotational axis A for wavelength scanning, the diffraction grating is shifted along the length of the groove (direction B) in synchronous with the rotational angle.
In any of the conventional methods, an auxiliary mechanism is needed besides that for rotating the diffraction grating. In the former case, for example, an appropriate mechanism is needed for properly arranging the auxiliary mirrors, and in the latter case, an appropriate mechanism is needed for moving the diffraction grating along the length of the grooves. In addition to that, a controller for synthesizing two mechanisms at high precision is required.
Another type of monochromator is proposed addressing the same problem in the U.S. Pat. No. 5,274,435, “Grating monochromators and spectrometers based on surface normal rotation” to M. C. Hettrick. In the monochromator, wavelength scanning is done by rotating the diffraction grating about an axis normal to the surface of the diffraction grating and standing at the incident point. In the patented monochromator also the diffraction efficiency is maximized at a certain wavelength, but is not maximized at the other wavelengths.
Thus a primary object of the present invention is to provide such a spectrometer or a monochromator based on surface normal rotation yielding the maximum diffraction efficiency at any wavelength. Another object of the present invention is to provide such a spectrometer with a minimized aberration. Still another object of the present invention is to provide a diffraction grating suited for use in such a spectrometer or a monochromator.